A child in Suifenhe, Heilongjiang province, receives a nucleic acid test on Friday. Officials are implementing contingency measures to prevent an outbreak. XU KAIXIN/XINHUA

Pidu district, Chengdu, Sichuan province, has begun free nucleic acid testing for all its residents after 10 confirmed COVID-19 cases and two asymptomatic cases were detected in the district, home to more than 800,000.

The latest case was announced by the provincial health authority on Friday－a 57-year-old woman in Xipu town, Pidu.

To detect potential cases in a timely manner and prevent a further outbreak, Pidu started districtwide universal COVID-19 testing at 6 pm on Friday, said a notice issued by the district's COVID-19 epidemic prevention and control command on Friday.

Numerous sites were set up to collect samples from both long-term and short-term inhabitants as well as students in the district, it said.

As all new cases were limited to Pidu, the scope of the COVID-19 outbreak in Chengdu did not expand further, said Zhu Xiaoping, a public health researcher working in the leading group coping with COVID-19 under the Sichuan Health Commission.

Notably, one of the confirmed cases, a 50-year-old migrant worker, turned out to be a close contact of a previously found asymptomatic patient, experts disclosed, and they called for improved intervention involving asymptomatic cases.

After the first case of COVID-19 was confirmed on Sunday, all those who had close contact with the individual were quarantined and given nucleic acid tests. All the ensuing positive cases but one in Chengdu were found among quarantined individuals in Pidu.

Pidu, commonly referred to as Pixian by people in Sichuan, became a district of Chengdu in 2016 when Pixian county was promoted to district-level status and changed its name to Pidu.

A 69-year-old woman surnamed Lu and her 71-year-old husband Zhao－who live in Taiping, a village in Pidu－were diagnosed as confirmed COVID-19 patients on Sunday and Monday.

Since then, all new infections have been found in Pidu. The couple's 20-year-old granddaughter, surnamed Zhao, is one of the confirmed cases.

Zhao, who lives in a rented apartment in a residential quarter of Chengdu's Chenghua district, became an online sensation because she had traveled extensively in different districts of Chengdu before she was diagnosed as a positive COVID-19 case.

Netizens disclosed Zhao's personal information including her name, ID number, family address and photos. The public harassment of Zhao drew the attention of Peng Qinghua, Party secretary of Sichuan, who requested that attention should be paid to the protection of patient privacy.

Nationwide, the National Health Commission said Friday that it received reports of 15 newly confirmed COVID-19 cases on the Chinese mainland on Thursday－six domestically transmitted cases and nine imported ones.

The city of Suifenhe, Heilongjiang province, reported two local asymptomatic cases of COVID-19 on Friday, according to the local government.

The two patients－a 32-year-old male worker and a 14-year-old male student－are close contacts of a confirmed case reported on Thursday.

Zhou Huiyingcontributed to this story.